Intentando evitar el cambio de orientación de la pantalla al salir de la actividad de inicio de sesión de Facebook en Android

To begin with, here is my AndroidManifest :

<application
    android:icon="@drawable/ic_launcher"  
    android:label="@string/app_name"  
    android:theme="@android:style/Theme.NoTitleBar.Fullscreen">  
    <meta-data android:name="com.facebook.sdk.ApplicationId" android:value="@string/app_id"/>  
    <activity
        android:name=".MainActivity"  
        android:label="@string/app_name"  
    android:screenOrientation="portrait"
            android:configChanges="keyboard|keyboardHidden|orientation|screenSize">  
            <intent-filter>  
                <action android:name="android.intent.action.MAIN" />  
                <category android:name="android.intent.category.LAUNCHER" />  
            </intent-filter>  
        </activity>  
        <activity android:name="com.google.ads.AdActivity"  
            android:screenOrientation="portrait"  
            android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"/>  
        <activity
            android:name="com.facebook.LoginActivity"
            android:screenOrientation="portrait"
            android:configChanges="keyboard|keyboardHidden|orientation">  
        </activity>  
    </application>

What I'm doing is this :
1. Show my MainActivity which contains a button to login with Facebook.
2. When the user clicks on the Facebook login button, the Facebook login activity is shown (instead of the MainActivity) asking the user for permissions among other things.
3. If the user decides to press the "back" key or the "cancel" button (so he can return to the MainActivity), the Facebook login dialog box disappears and then the status bar (I'm using the Samsung galaxy tab 10.1) goes into landscape mode for no reason and goes back to portrait mode just before showing the MainActivity.

I can't find a reason why it does that, it's really annoying. If anyone could help me with this, that would be great.

Gracias por adelantado